The present invention relates generally to an incident radiation absorber and more particularly to an improved high power laser dump.
Laser system applications often require that the laser beam be reshaped. In so doing, some of the rays of the laser beam are separated or stripped from the beam of interest and the energy contained therein must be safely dissipated. In high power laser systems, this energy is substantial and the construction of a suitable heat absorbing device, commonly known as a laser dump, can present formidable problems.
Various geometric configurations of such laser dumps have been proposed and constructed but each have certain undesirable characteristics which limit their effectiveness.
Most existing laser dumps, for example, utilize a liquid coolant to absorb the incident radiant energy to keep the temperature of the dump material within allowable working limits. Liquid cooling requires complex cooling channel networks and associated piping connections for the coolant. To increase the heat transfer rate, the coolant is usually forced through the cooling channels at a very high velocity under great pressure, which necessitates the use of a high pressure pump.
In many high energy laser devices, for example the chemical lasers, the distribution of flux energy in the beam is not uniform. Peak fluxes may be five or more times greater than the average flux. Consequently, at the peak flux spots, the heat input to the laser dump is so high that the coolant may begin to boil. Boiling will cause a decrease in the heat transfer rate, resulting in a rapid increase of the laser dump wall temperature and ultimate material failure.
Some laser dumps, such as those described in U.S. Pat. Nos. 4,267,523 and 4,271,396, both issued to Donald G. Brown on May 12, 1981 and June 2, 1981 respectively, utilize a coolant fluid in conjunction with improved laser dump structure. In U.S. Pat. No. 4,267,523, laser energy is directed to water-cooled panels arranged to form a generally hexagonally-shaped cavity. A laser beam enters the cavity via a longitudinal opening therein and at an acute angle with respect to the plane of the first panel, but normal to the longitudinal axis of the cavity. Thus incident energy is partially absorbed by the first panel and because of the angular relationship between the incoming radiation and the panels, is continuously directed to the other panels until it exits the cavity at a greatly diminished energy level. In U.S. Pat. No. 4,271,396 the absorption cavity is substantially cylindrical in shape such that the incident energy is confined within the cavity. Here again, however, the incident beam is apparently normal to the axis of the cavity and as such, heat absorption is limited to specific longitudinal areas of the cavity. Moreover the absorption material is apparently uniform throughout the cavity.
In another laser dump, that utilizes a cooling fluid, the laser beam is caused to shine into the open end of a hollow cylinder along the longitudinal axis of the cylinder. The beam strikes a somewhat polished, non-perpendicular bottom surface where a small percentage of the energy is absorbed. The remaining energy is then reflected upwardly toward the inner wall of the cylinder. Again, some of the energy is absorbed in the wall of the cylinder and the remainder is reflected upwardly to impinge upon a larger area on the diametrically-opposed surface of the cylinder. This partially absorbing-partially reflecting process continues until the remaining energy shines out of the open top surface of the cylinder. A particular disadvantage of the device described above is that the beam finally does emerge from the cylinder as backscattering and, although greatly reduced in energy level, presents a potential hazard to the environment and personnel therein. Also, the required length of the cylinder to dissipate energy is substantial.
Another form of laser heat dump is manufactured by Schafer Associates of Wakefield, Mass. In this device, laser energy is directed to a pointed reflective surface which reflects the energy to an absorber in the shape of a hemispherical dome. Such a device has an inherent weakness at the apex of the reflector which is subjected to great heat. Furthermore, the volume of such a device is quite large.